The invention disclosed herein relates to active electrical non-linear circuits in general, and to analog frequency multipliers in particular. A variety of methods common in the art utilize a multiplying or modulating device in order to obtain the product of two or more time varying signals. This product contains both the sum and the difference of the input frequencies, commonly known as sidebands. In the field of audio signal processing, these sidebands are typified as harmonics or partials wherein the upper sideband is a multiple of an input frequency. In radio circuits, frequency selective filter circuits are typically employed to eliminate the unwanted sideband. It is usually desirable to drive the multiplying device with two signals that are in phase quadrature, thus necessitating the use of additional frequency selective phase shifting circuitry. Both of these practices necessarily limit the usable bandwidth of the circuit, and become a source of error as frequency approaches the skirts of the passband. Furthermore, most commonly employed frequency multiplication means are not well suited to applications in which the input signal amplitude varies to any significant degree. Means have been proposed in the prior art to accommodate variations in amplitude by normalizing the amplitude of the input signal, using dynamic compression for example, prior to multiplication. This method proves to be problematic for signal processing applications in which it is necessary to respond to and reproduce the dynamic range of the input signal. A further issue is in regard to a DC offset voltage that may be generated in the multiplication process. Typically, a coupling capacitor or high pass filter is employed at the output of the multiplier for the purpose of blocking the DC voltage. However, this adds the further complication of attenuating low frequencies and slowing circuit response to rapid changes in signal amplitude.
Means for generating a series of harmonics are also common in the related art. Most common among these are circuits which impress limits on the amplitude of a sinusoidal signal by way of clipping. Methods for generating even order harmonic distortion often employ half-wave or full-wave rectification of an input signal. Control of the amplitude of the harmonic products relative to the amplitude of the fundamental input signal is most commonly effected by changing the gain of the input signal while maintaining a fixed amplitude limit, or by changing the limit relative to the amplitude of the input signal. However, the relative amplitudes of each of the frequency domain formants which comprise the distorted waveform reside in fixed ratios that are predetermined by the nature and construction of the distorting circuit. The amplitudes of these formants are not individually adjustable over any broad span of frequencies in the analog domain. Changes in the relative balance of harmonics generated in this fashion can only be effected as a function of frequency, by frequency selective filter circuits. The constraining consequence of this technique is the lack of control over harmonic response independent of frequency response.
The various means recited in the prior art each exhibit one or more of the difficulties as described above. It is apparent in the detailed descriptions and accompanying figures which follow that the present invention circumvents each of these difficulties, and provides significantly simpler means that are distinct from and not reliant upon, the means revealed in the prior art.